This invention relates to the use of mordenite zeolites as catalysts in the monoalkylation of aromatic compounds to produce cumene, ethylbenzene, and other alkylated benzenes.
Cumene, also known as isopropylbenzene, is useful for the production of phenol, acetone, and alphamethylstyrene. Ethylbenzene is useful in the production of styrene. Various processes for their manufacture are known.
Various processing schemes comprising alkylation and/or transalkylation are known to produce monoalkylaromatic products such as cumene or ethylbenzene in high yields. However, existing processes are not without problems including the production of undesirable by-products. Examples of such by-products produced in conjunction with cumene include alkylating agent oligomers, heavy polyaromatic compounds and unwanted monoalkylated and dialkylated compounds such as n-propylbenzene, butylbenzenes and ethylbenzene. The production of unwanted xylenes are a particular problem in the production of ethylbenzene. Another problem with existing processes concerns the use of Friedel Crafts catalysts such as solid phosphoric acid or aluminum chloride. The phosphoric acid catalysts generally require the use of a water co-feed which produces a corrosive sludge by-product. Problems concerning the sludge by-product can be avoided by the use of zeolite catalysts. However, major drawbacks of the zeolite catalyzed gas phase processes include the production of undesirable by-products and the relatively rapid deactivation of the catalyst.
It is known that aromatic hydrocarbons can be alkylated in the presence of acid-treated zeolite. U.S. Pat. No. 4,393,262 (1983) teaches that cumene is prepared by the alkylation of benzene with propylene in the presence of a specified zeolite catalyst. U.S. Pat. No. 3,140,253 (1964) and U.S. Pat. No. 3,367,884 (1968) broadly teach the use of acid-treated mordenite for the alkylation of aromatic compounds. However, such alkylations are generally not selective with respect to site and number of substitutions. Further, catalysts are often quickly deactivated requiring timely and costly replacements or reactivation.
Important criteria which determine the feasibility of a commercial process for the alkylation of benzene or substituted benzene, besides the above-mentioned criteria regarding environmental impact, low level of impurities which are difficult to remove from the process stream and/or to convert to desired products, activity and stability of the catalyst, include the following: the conversion based on the alkylating agent should be substantially 100 percent in order to prevent expensive recycling of unconverted alkylating agent or loss of alkylating agent to flare: the selectivity towards the monoalkylated benzene should be high: the recycle of benzene or substituted benzene should be minimized as it is rather expensive and leads to the larger quantities of impurities which are recycled to the alkylation reactor and passed over the catalyst. Minimizing the recycle of benzene or substituted benzene could be achieved by decreasing the molar ratio of benzene or substituted benzene to alkylating agent. However, it is known that the conventional zeolite type catalysts tend to deactivate under these conditions because of increased tendency of the alkylating agent to polymerize and because of the increased polyalkylation activity. Apart from catalyst deactivation this, moreover, would lead to a decreased selectivity towards the monoalkylated benzene. Therefore, the main object of the conventional zeolite catalyzed processes was to optimize conversion of the alkylating agent and selectivity towards the monoalkylated product in the alkylation step or process in itself.
Thus, there remains a need for an effective process for the preparation of monoalkylated benzenes having minimal levels of impurities utilizing a catalyst having low negative environmental impact and long life.